1. Field of the Invention
The present invention relates to projection aligners in photolithography processes for use in the manufacture of semiconductor devices.
2. Description of the Background Art
Photolithography processes are adopted for various steps of the manufacture of semiconductor devices. In the photolithography processes, a projection aligner is employed for defining various patterns on a semiconductor wafer.
In the projection aligner, the intensity of the light applied to a photoresist disposed above the semiconductor wafer is controlled by a photomask (reticle).
FIG. 12 is a plan view of a conventional photomask 9. FIG. 13 is a cross section schematically illustrating the structure taken along the line B1--B1 in FIG. 12. The photomask 9 comprises a body 92 composed of quartz, and shields 93 selectively provided on the body 92. The shields 93 are formed from chromium (Cr), for example, and they considerably reduce the light transmittance with respect to the position where only the body 92 exists.
In order to raise resolution limit due to miniaturization, there have been proposed phase shift masks such as Levenson type photomasks. FIG. 14 is a plan view of a conventional Levenson type photomask 90. FIG. 15 is a cross section schematically illustrating the structure taken along the line B2--B2 in FIG. 14. The photomask 90 has three regions 90a, 90b and 90c. The phase of the light passing through the region 90c is shifted .pi. with respect to the phase of the light passing through the region 90b. The region 90a reduces significantly the light transmittance with respect to the regions 90b and 90c.
The photomask 90 comprises a body 92, shields 93 selectively provided on the body 92, and phase shifters 94. The shields 93 open at the position corresponding to the region 90a or 90b, and the phase shifters 94 cover the opening of the shields 93 at the position corresponding to the region 90c. The photomask 90 can control both the intensity and phase of the light passing therethrough, thereby to optimize resolution and depth of focus.
Besides the stated technique, light source of shorter wavelength, modified illumination obtained by control of an aperture, and pupil filter method have been proposed, in order to raise resolution limit due to miniaturization. The pupil filter method comprises disposing a patterned spatial frequency filter at the pupil surface in a projection lens system. The spatial frequency filter can also control both the intensity and phase of the light passing therethrough, thereby to optimize resolution and depth of focus. It is however noted that the light passing through the projection lens system varies according to the pattern of the photomask 90. It is therefore preferable to optimize the pattern of a spatial frequency filter for every photomasks.
The photomask 90, however, calls for patterns which are independent one another in their respective steps of the manufacture of semiconductor devices, and there are also needed various mask making processes such as electron beam cutting, separating from wafer processes. Further, since the phase shift mask calls for the phase shifter 94 in addition to the body 92 and shields 93, its design, making control and defect inspection are complicated. This causes an increase in the photomask revision. Consequently, the conventional photomasks have required a considerable amount of time for their making.
The projection lens system is the essential part in order that a photomask pattern is faithfully reproduced on a wafer, and it is necessary to maintain a high precision. The projection lens system is therefore placed in a closed case, in order to assure the rigid control of temperature, humidity and pressure. Since the spatial frequency filter is a component of the projection lens system, this cannot be replaced easily as is the case with aperture. It is therefore difficult to control the pattern of a spatial frequency filter for every photomasks.